1. Field of the Invention
The present invention relates generally to a piezoelectric transformer, a manufacturing method and a driving method thereof. The invention relates to a piezoelectric transformer to be employed in a deflector device of a television receiver, a charge device of a copy machine and so forth, and a manufacturing method and driving method thereof.
2. Description of Related Art
In general, in a power source circuit in an apparatus which requires high voltage, such as a deflector device of a television receiver, a charge device of a copy machine and so forth, a winding type electromagnetic transformer have been employed as a transformer element for generating high voltage. The electromagnetic transformer has a construction, in which a conductive wire is wound around a core of a magnetic body, Therefore, in order to realize a high transformation ratio, a number of turns of the conductive wire wound around the core has to be increased. Therefore, it is quite difficult to realize a compact and thin electromagnetic transformer.
On the other hand, there has been proposed a piezoelectric ceramic transformer (hereinafter referred to as a "piezoelectric transformer") employing piezoelectric effect. Such a piezoelectric transformer has been used in a resonating condition. In comparison with the electromagnetic transformer, (1) wiring structure is unnecessary and energy density is high, the transformer can be made compact and thin, (2) the transformer can be made non-inframable, and (3) noise due to electromagnetic induction or so forth is not present, and large number of other advantages are present.
However, on the other hand, since the piezoelectric transformer operates in a resonating condition, connecting structure between respective components of the transformer, particularly an extracting structure of a terminal (external terminal) for external electrical connection, has to have a structure which can guarantee high reliability of connection to withstand against vibration associated with transforming operation.
In view of reliability of connection set forth above, a Roazen type piezoelectric transformer as one example of the piezoelectric transformer is considered. FIG. 13 is a perspective view showing a construction of the Roazen type piezoelectric transformer. FIG. 14A is a section taken along line L--L along a longitudinal direction of the transformer of FIG. 13 and a diagrammatic illustration showing a connecting condition with the external terminal.
Referring to FIG. 13, at both ends in the longitudinal direction of a piezoelectric plate in elongated plate structure, two driving portions 91a and 91b having low impedance are provided respectively. On the upper surface of the driving portion 91a at the left side in the drawing, an electrode 912 is provided. On the lower surface, an electrode 913 is provided. Similarly, on upper and lower surfaces of the driving portion 91b at the right side, electrodes 915 and 916 are provided.
In these two driving portions 91a and 91b, as shown in FIG. 14A, both of upper electrodes 912 and 915 are connected to an external terminal 917 and both of lower electrodes 913 and 916 are connected to an external terminal 918. Both of piezoelectric plates 911 and 914 at the driving portions 91a and 91b are polarized in the direction of thickness as shown by arrows Ya and Yb in the drawing.
In the portion positioned at the center portion located between two driving portions 91a and 91b, a generator portion 92 having high impedance, is present. At the center of the generating portion 92, a strip-shaped electrode 923 is provided surrounding the circumference of the piezoelectric plate at this portion. The electrode 923 is connected to an external terminal 919 (see FIG. 14A). The piezoelectric plates 921 and 922 at portions between the electrode 923 and two driving portions 91a, 91b, respectively, are oppositely polarized in the longitudinal direction across the electrode 923 as shown by arrows Yc and Yd in the drawing.
When a high voltage is taken from this piezoelectric transformer, the transformer operates as follows. When an alternating voltage is applied to the electrodes 912 and 913 from external terminals 917 and 918 in case of the driving portion 91a, (915 and 916 in case of the driving portion 91b), at the driving portion 91, an alternating current voltage field is applied in the polarizing direction of the piezoelectric plate 911 (or 914). Then, by a so-called piezoelectric transverse effect 31 mode in which displacement in the perpendicular direction relative to the polarizing direction of the piezoelectric plate is caused by the alternating current voltage field, longitudinal vibration is applied to the piezoelectric plate 921 (or 922) in the longitudinal direction to cause vibration on the overall transformer.
On the other hand, in the generating portion 92, in response to longitudinal vibration in the longitudinal direction, by a so-called piezoelectric longitudinal effect 33 mode in which potential difference is generated in the polarizing direction due to mechanical distortion caused in the polarizing direction of the piezoelectric plate, a voltage is generated between the electrodes 923 and 912 (or 915) or between the electrodes 923 and 913 (or 916). This voltage is taken as the output voltage from the external terminal 919. At this time, when the frequency of an alternating drive voltage applied between the external terminals 917 and 918 is set equal to the resonance frequency of the piezoelectric transformer, quite high output voltage can be obtained.
It should be noted that when a low voltage is to be output by inputting high voltage, the high impedance generating portion 92 causing the piezoelectric longitudinal effect is taken as an input side and low impedance driving portion 91 causing piezoelectric transverse effect is taken as an output side for operation.
Here, concerning reliability of connection, discussion will be given for mounting positions of the external terminals 917, 918, 919 in the Roazen type piezoelectric transformer to respective electrodes 912, 913, 915, 916 and 923.
Referring to FIG. 14A, the terminal 917 at the driving portion side is connected to the electrode 912 in the driving portion 91a and the electrode 915 in the driving portion 91b via junctions 911a and 913c, respectively. Similarly, another terminal 918 at the driving portion side is connected to the electrode 913 in the driving portion 91a and the electrode 916 in the driving portion 91b via junctions 912a and 914c, respectively. On the other hand, the terminal 919 at the generator portion side is connected to the electrode 923 in the generator portion 91 via a junction 921b.
Here, with reference to FIG. 14B illustrating distribution of displacement in the longitudinal direction in longitudinal vibration tertiary mode in the length of the transformer, this mode is a vibration mode, in which 3/2 wavelength of the longitudinal vibration is equal to the entire length of the piezoelectric transformer, in which three nodes of vibration N1, N2 and N3 and four peaks of vibration L1, L2, L3, L4 are present. Comparing the positions of the nodes of vibration and the positions of respective junctions of the external terminals, the junctions 911a and 912a can be matched with the node N1, the junctions 921b can be matched with the node N2 and the junctions 913c and 914c are matched with the node N3. Namely, this type of piezoelectric transformer can arrange all of the junctions for connection of the external terminals at the nodes of vibration to assure good vibration characteristics and reliability of connection.
On the other hand, considering that the piezoelectric transformer is designed to extract high voltage, in view of security of the equipment to which the transformer is assembled, such as high voltage generating power source circuit and so forth and freedom of circuit design, the terminal structure is desirable to be a four terminal structure, namely, a pair of input terminals and a pair of output terminals, and thus four terminals in total are isolated from one another. However, in the conventional piezoelectric ceramic transformer, it is difficult to achieve both of the foregoing two points.
Namely, in the conventional piezoelectric transformer, there is only one electrode 923 for extracting generated voltage from the generator portion 92. In other words, the output side terminal is only one terminal 919. This type of transformer is basically a three terminal structure. Accordingly, upon practical use, one of two terminals 917 and 918 at the driving portion side has to serve as a common terminal for input and output.